I'm a little curious why Rick Smith, the general manager, did not fire the previous staff. It seems like making the new guy do all the dirty work, but then again, maybe that is the established power structure on Kirby. They want a strong head coach.

Yep. You answered your own question: they didn't fire the coaching staff because they wanted the new head coach to do it. That, in turn, sends a signal that the new coach means business.

This is more than a coaching change; this is a culture change. McNair is not playing around (even his attitude seems different) and wants to send a strong signal. You do that by hiring a head coach that's not afraid to roll in and clean house.